1. Field of the Invention
The present invention relates to a polymer scale preventive agent useful in polymerization of a monomer having an ethylenically unsaturated double bond, a polymerization vessel effective in preventing polymer scale deposition, and a process of producing a polymer using said vessel.
2. Description of the Prior Art
As a method of polymerizing a monomer having an ethylenically unsaturated double bond, are known suspension polymerization, emulsion polymerization, solution polymerization, gas phase polymerization and bulk polymerization. In any type of the polymerizations, polymer scale is liable to be deposited on the areas with which the monomer comes into contact during polymerization, such as inner walls, stirring equipment and so on of a polymerization vessel.
The deposition of the polymer scale results in disadvantages that the yield of the polymer and cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into a polymeric product, thereby impairing the quality of formed products obtained by processing the polymeric product; and that removal of such polymer scale is laborious and hence time-consuming. Further, the polymer scale contains unreacted monomers and operators may be exposed thereto, which may cause physical disorders in the operators.
Heretofore, as a method for preventing polymer scale deposition on the inner wall and so forth, there have been known a number of methods, for example, a method in which a polar organic compound such as amine compounds, quinone compounds, aldehyde compounds, etc. is preliminarily applied to the inner wall, etc. of a polymerization vessel, as is carried out in some instances of suspension polymerization of vinyl chloride, or a method in which such compounds are added to an aqueous medium for suspension polymerization (Japanese Patent Publication (KOKOKU) No. 45-30343 (1960)).
These methods can prevent the deposition of polymer scale if polymerization run is repeated within about 5 or 6 times. However, if the number of repetition of polymerization run exceeds 5 or 6, the scale preventive effect according to the above methods is weakened. That is, the scale preventive effect is poor in durability. This disadvantage is emphasized particularly where a water-soluble catalyst is used. Thus, the polymer scale prevention according to the above-mentioned methods is unsatisfactory industrially.
It is proposed in Japanese Pre-examination Patent Publication (KOKAI) No. 53-13689 (1978) to form a coating of a condensation product of an aromatic amine compound on the areas with which monomers come into contact, such as the inner wall of a polymerization vessel. The formation of the coating of such a condensation product enables repetition of about 100 to 200 polymerization runs without deposition of polymer scale on the areas in the liquid phase, i.e., under the liquid surface inside the polymerization vessel. The effect of preventing the polymer scale deposition on the areas in the liquid phase is achieved even where a water-soluble catalyst is used.
However, the scale preventing method using a condensation product of aromatic amine compound has the drawback that polymer scale deposition is liable to occur in the vicinity of the interface between the gas phase and the liquid phase which is located at the upper section of a polymerization vessel.
Once polymer scale is deposited in the vicinity of the interface between the gas phase and the liquid phase, the deposited scale will grow gradually as polymerization runs are repeated, and at last it peels off from the inner wall, etc. to be incorporated into the polymeric product. If the polymeric product containing the polymer scale is processed into formed products such as sheets or the like, the polymer scale causes increase in fish eyes in the formed products, thereby lowering seriously the quality of the products.
Besides, where polymeric products obtained by polymerization are processed into formed products such as sheets, etc., the formed products are required to have a high whiteness. That is, when a polymeric product is formed into a sheet or the like without any addition of a coloring agent, the resulting formed product is more or less colored. Such coloration is called initial coloration, which is desired to be as low as possible. However, the coating comprising said condensation product of an aromatic amine compound disclosed in the Japanese Pre-examination Publication may be peeled or dissolved into a polymeric product, thereby lowering the whiteness, or increasing the initial coloration, of the formed products.
Furthermore, in forming a coating of the condensation product of the aromatic amine compound as described above, the condensation product is dissolved in a solvent to prepare a coating solution. The solvent is an organic solvent or a mixed solvent of water and an organic solvent, the mixed solvent being based on the organic solvent and normally containing the organic solvent in a concentration of 60% by weight or above. Therefore, there is a danger of organic-solvent inflammation, explosion or the like, together with safety problems in handling thereof, such as toxicity.